1. Technical Field
The invention relates to a method and system for reserving transmission capacity required by the acknowledgement and/or response messages in bi-directional transmission systems.
2. Discussion of Related Art
Many modern transmission systems use time division multiple access technology utilizing one available resource in order to transmit information which is obtained from several different sources. Such systems often include a so called point-to-multipoint architecture, a possibility to transmit packets in the form of bursts, and a centralized management of the transmission resource, such as a transmission channel. For instance transmission systems using the the Transmission Control Protocol (TCP) can be realized as systems of the mentioned type.
FIG. 1 shows a prior art solution based on the so-called reservation contest method for reserving transmission capacity in systems of the above-described kind. The transmission system 10 according to FIG. 1a consists of a transmission network 11 and four transmission units 12, 13, 14 and 15 connected to the network 11. When an information packet 16 shall be transmitted from the first unit 12 to the second unit 13 the information packet 16 is transmitted directly to the transmission network 11. If the transmission capacity of the network 11 is available the information packet 16 is conveyed from the first unit 12 to the second unit 13, as shown in FIG. 1a. If for instance the third unit 14 simultaneously tries to transmit another information packet 17, there occurs a so-called collision between the packets, as shown in FIG. 1b. In such a situation neither transmitted packet 16 and 17 will reach its destination.
When the first and third transmission units 12 and 14 detect the collisions, for instance by listening to the traffic in the network 11, they both wait a period of a random length, and then they try to re-transmit the packets 16 and 17. This is continued until the transmission of the packets succeeds without collisions.
When the above described reservation contest method is used it is natural that also the probability of collisions increases when the number of information packets moving in the network 11 increases. For instance, the theoretical maximum efficiency, or the throughput probability of the transmission, in the widely used slotted ALOHA reservation contest system is 1/e≈37% [1] in a system with infinitely many users. This restricts the usefulness of reservation contest methods in congested networks.
FIG. 2 illustrates the use of the above described prior art reservation contest method in the transmission between the first and second transmission units 12 and 13, when the first unit 12 transmits information 16 which requires an acknowledgement (ACK) to the second unit 13. This is the case for instance in transmitting a simple information packet according to the TCP, because according to the TCP each received packet must be acknowledged, otherwise the transmitting unit thinks that the packet could not reach its destination and tries to re-transmit the packet.
In FIG. 2, as in the other diagrams in this patent application illustrating the information travel, the time axis extends from top to bottom, in other words, the events which are higher up occur before those events which are below them.
When the first transmission unit 12 has successfully transmitted the information packet 16 through the network 11 to the second unit 13 the second unit 13 tries to acknowledge that the packet 16 is received, by transmitting via the network 11 an acknowledgement signal 21 to the first unit 12. As was noted above when the reservation contest method is used, due to the packet collisions it may become necessary that the acknowledgement signal is transmitted many times before the first unit receives the acknowledgement. This means unnecessary consumption of resources both in the second unit 13 and in the network 11, compared to a situation where the acknowledgement signal 21 is conveyed to the first unit 12 by a single transmission.
FIG. 3 shows another prior art embodiment 30 for reserving transmission capacity in a transmission system. The embodiment is based on a resource management table maintained in the first transmission unit 12, where the available transmission capacity is allocated to different units on the basis of request provided by them. In FIG. 3 a message 16 is transmitted from the first unit 12 to the second unit 13. If the message is for instance a status inquiry or any such information which requires that a response message 33 be transmitted back to the first unit 12, then the second transmission unit 13 recognizes the need for a response message when it processes the received packet 16.
The second transmission unit 13 transmits a capacity request 31 to the first unit 12 so that it is able to reserve the transmission capacity required to transmit the response message 33. Because the second unit 13 is not able to control the transmission resource in use, the reservation request 31 must by transmitted with the aid of the reservation contest method described above. The first unit 12 processes the capacity request 31 and updates the granted capacity in the resource distribution table. Then the first unit 12 transmits the information about the transmission capacity reserved for the transmission of the response message 33 with the aid of a grant message 32. The second unit 13 receives the grant message 32 and on the basis of the information contained in it the second unit transmits the response message 33 to the first unit 12 with the aid of the transmission capacity allocated to the second unit by the first unit 12.
As such the prior art solution 30 shown in FIG. 3 is more advanced than the system based on reservation contest presented in FIG. 2, but even the system 30 according to FIG. 3 utilizes the transmission capacity at a rather low efficiency, because also the transmission of the response message 33 requires the transmission of two other messages, i.a. the request and grant messages 31 and 32. Further the system according to the FIG. 3 is not able to eliminate the use of the reservation contest system completely, because the reservation request message 31 is still transmitted by the reservation contest system.
The object of this invention is to provide a new method and system with which the
According to a first aspect of the invention, a method for reserving transmission capacity required by acknowledgement and/or response messages in a bi-directional transmission system, where a message is transmitted form a first unit to a second unit, a second unit receives the message transmitted by the first unit, and the second unit transmits an acknowledgement and/or response message as a response to the received message, is characterized in that a reservation of transmission capacity required by the acknowledgement and/or response message is made substantially in connection with transmission of the message transmitted from the first unit to the second unit.
Further in accord with the first aspect of the invention, the reservation of the transmission capacity required by the acknowledgement and/or response message is made before said message is transmitted from the first unit to the second unit.
Still further according to the first aspect of the invention, the reservation of the transmission capacity required by the acknowledgement and/or response message is made simultaneously when said message is transmitted from the first unit to the second unit.
Still further according to the first aspect of the invention, the reservation of the transmission capacity required by the acknowledgement and/or response message is made after said message has been transmitted from the first unit to the second unit.
Further still in accordance with the first aspect of the invention, the bi-directional transmission system utilizes the TCP transmission protocol.
Still further in accordance with the first aspect of the invention, the second unit transmit an acknowledgement and/or response message to the first unit.
Yet further in accordance with the first aspect of the invention, in addition to the first unit and the second unit, the transmission system contains at least a third unit, wherein the second unit transmits the acknowledgement and/or response message to the third unit.
Further in accordance with the first aspect of the invention, the bi-directional transmission system is a cable television system or a wireless point-to-multipoint system.
According to a second aspect of the invention, a system for reserving transmission capacity required by the acknowledgement and/or response messages in a bi-directional transmission system, wherein the system comprises at least a first unit for transmitting and receiving a message, a second unit having means for transmitting acknowledgement and/or response messages, is characterized in that the first unit or the second unit has means for reserving the transmission capacity required by the acknowledgement and/or response message substantially in connection with the transmission of a message transmitted from the first unit to the second unit.
Further in accord with the second aspect of the invention, the bi-directional transmission system is a cable television system or a wireless point-to-multipoint system.
With the solution according to the invention the available transmission capacity can be better utilized than in a prior art reservation contest method, as the re-transmission caused by packet collisions are avoided.
With the solution according to the invention it is further possible to reduce the communication between different units, for instance compared to cases according to the FIG. 3, where the unit which transmits a response packet must reserve the transmission capacity required for the response via a separate request operation. The solution according to the invention requires no corresponding request operation, whereby the transmission capacity required by the request operation becomes available to the actual data transmission. At the same time also the use of the reservation contest method required by the request message is avoided.
The solution according to the invention can be realized by software changes, and thus it will not essentially increase the manufacturing costs of the used system.